A common special effect in film and television production is the composition of a foreground visual element (an actor, for example) over a different background to produce a combined, or composite picture. Often this technique is used when it is impractical to have all subjects in the scene during shooting, for safety or economic reasons, or when some of the elements are actually miniature models.
Sometimes it is possible to film the foreground subject in front of a controlled special background color, in which case other schemes for electronically combining foreground and background images are known (see, for example, U.S. Pat. No. 4,100,569). Since precise control of the foreground environment is not always possible or practical, rotoscoping techniques are often used to generate the outline of the foreground element. The rotoscoping technique typically involves the display of the original picture containing the subject, and allows the operator to draw or otherwise construct a close outline around the visual elements. These regions are then filled using some appropriate technique to generate matte elements. Depending upon the requirements of the subsequent process, the regions may be filled either inside or outside to create either a matte which is black where the foreground element is, and clear everywhere else (a "positive", "male", or "clear-core" matte), or a matte which is clear where the foreground element is and black everywhere else (a "negative", "female", or "black-core" matte).
This technique historically has been carried out using paint on film, and modern manifestations of this same technique often use computers with graphics displays to show the foreground picture and permit the operator to create the outline. With computers, digital representations corresponding to the inside and outside of the outline may be created (and the actual colors may be arbitrary). Intermediate values may be used to specify anti-aliased pixels along the outline edges to give the illusion of a smooth outline. Furthermore, by using the computer for rotoscoping a sequence of frames, the operator may be able to specify outlines for only certain keyframes, and the computer can generate the outlines and mattes for the in-between frames automatically.
One of the deficiencies of the prior technique is that it does not work well for certain types of visual elements which do not have a distinct boundary. Semi-transparent edges occurring in hair, clouds, or blurred objects in motion or out of focus are particularly difficult. Other visual elements which exhibit soft edges, such as shadows, present similar difficulties for matte extraction by prior rotoscoping methods. One method which attempts to overcome the problems presented by the soft-edge areas is general blurring of the matte. A hard-edge matte is first constructed and then blurred using image-processing or optical methods to create a matte with a soft edge. The problem with this technique, however, is that the element usually requires different degrees of softness along its outline, and general blurring of a hard-edge matte cannot simply meet this requirement. It is therefore desirable to overcome the shortcomings of prior matte extraction techniques by providing control for generating soft-edge mattes.